A BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach

We study the spontaneous symmetry breaking of the excitonic insulator state induced by the Coulomb interaction $U$ in the two-dimensional extended Falicov-Kimball model. Using the variational cluster approximation (VCA) and Hartree-Fock approximation (HFA), we evaluate the order parameter, single-particle excitation gap, momentum distribution functions, coherence length of excitons, and single-particle and anomalous excitation spectra, as a function of $U$ at zero temperature. We find that in the weak-to-intermediate coupling regime, the Fermi surface plays an essential role and calculated results can be understood in close correspondence with the BCS theory, whereas in the strong-coupling regime, the Fermi surface plays no role and results are consistent with the picture of BEC. Moreover, we find that HFA works well both in the weak- and strong-coupling regime, and that the difference between the results of VCA and HFA mostly appears in the intermediate-coupling regime. The reason for this is discussed from a viewpoint of the self-energy. We thereby clarify the excitonic insulator state that typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate of preformed excitons (strong-coupling regime).Comment: 11 pages, 9 figure

Affective compatibility between stimuli and response goals: a primer for a new implicit measure of attitudes

We examined whether a voluntary response becomes associated with the (affective) meaning of intended response effects. Four experiments revealed that coupling a keypress with positive or negative consequences produces affective compatibility effects when the keypress has to be executed in response to positively or negatively evaluated stimulus categories. In Experiment 1, positive words were evaluated faster with a keypress that turned the words ON (versus OFF), whereas negative words were evaluated faster with a keypress that turned the words OFF (versus ON). Experiment 2 showed that this compatibility effect is reversed if an aversive tone is turned ON and OFF with keypresses. Experiment 3 revealed that keypresses acquire an affective meaning even when the association between the responses and their effects is variable and intentionally reconfigured before each trial. Experiment 4 used affective response effects to assess implicit in-group favoritism, showing that the measure is sensitive to the valence of categories and not to the valence of exemplars. Results support the hypothesis that behavioral reactions become associated with the affective meaning of the intended response goal, which has important implications for the understanding and construction of implicit attitude measures

Theory of the waterfall phenomenon in cuprate superconductors

Based on exact diagonalization and variational cluster approximation calculations we study the relationship between charge transfer models and the corresponding single band Hubbard models. We present an explanation for the waterfall phenomenon observed in angle resolved photoemission spectroscopy (ARPES) on cuprate superconductors. The phenomenon is due to the destructive interference between the phases of the O2p orbitals belonging to a given Zhang-Rice singlet and the Bloch phases of the photohole which occurs in certain regions of k-space. It therefore may be viewed as a direct experimental visualisation of the Zhang-Rice construction of an effective single band model for the CuO2 plane.Comment: 11 pages, 9 Postscript figure

Self-energy and Fermi surface of the 2-dimensional Hubbard model

We present an exact diagonalization study of the self-energy of the two-dimensional Hubbard model. To increase the range of available cluster sizes we use a corrected t-J model to compute approximate Greens functions for the Hubbard model. This allows to obtain spectra for clusters with 18 and 20 sites. The self-energy has several `bands' of poles with strong dispersion and extended incoherent continua with k-dependent intensity. We fit the self-energy by a minimal model and use this to extrapolate the cluster results to the infinite lattice. The resulting Fermi surface shows a transition from hole pockets in the underdoped regime to a large Fermi surface in the overdoped regime. We demonstrate that hole pockets can be completely consistent with the Luttinger theorem. Introduction of next-nearest neighbor hopping changes the self-energy stronlgy and the spectral function with nonvanishing next-nearest-neighbor hopping in the underdoped region is in good agreement with angle resolved photoelectron spectroscopy.Comment: 17 pages, 18 figure

Mott transition and ferrimagnetism in the Hubbard model on the anisotropic kagom\'e lattice

Mott transition and ferrimagnetism are studied in the Hubbard model on the anisotropic kagom\'e lattice using the variational cluster approximation and the phase diagram at zero temperature and half-filling is analyzed. The ferrimagnetic phase rapidly grows as the geometric frustration is relaxed, and the Mott insulator phase disappears in moderately frustrated region, showing that the ferrimagnetic fluctuations stemming from the relaxation of the geometric frustration is enhanced by the electron correlations. In metallic phase, heavy fermion behavior is observed and mass enhancement factor is computed. Enhancement of effective spatial anisotropy by the electron correlations is also confirmed in moderately frustrated region, and its effect on heavy fermion behavior is examined.Comment: 5 pages, 6 figure

Landau mapping and Fermi liquid parameters of the 2D t-J model

We study the momentum distribution function n(k) in the 2D t-J model on small clusters by exact diagonalization. We show that n(k) can be decomposed systematically into two components with Bosonic and Fermionic doping dependence. The Bosonic component originates from the incoherent motion of holes and has no significance for the low energy physics. For the Fermionic component we exlicitely perform the one-to-one Landau mapping between the low lying eigenstates of the t-J model clusters and those of an equivalent system of spin-1/2 quasiparticles. This mapping allows to extract the quasiparticle dispersion, statistics, and Landau parameters. The results show conclusively that the 2D t-J model for small doping is a Fermi liquid with a `small' Fermi surface and a moderately strong attractive interaction between the quasiparticles.Comment: Revtex file, 5 pages with 5 embedded eps-files, hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to: [email protected]

Anomalous Spin and Charge Dynamics of the 2D t-J Model at low doping

We present an exact diagonalization study of the dynamical spin and density correlation function of the 2D t-J model for hole doping < 25%. Both correlation functions show a remarkably regular, but completely different scaling behaviour with both hole concentration and parameter values: the density correlation function is consistent with that of bosons corresponding to the doped holes and condensed into the lowest state of the noninteracting band of width 8t, the spin correlation function is consistent with Fermions in a band of width J. We show that the spin bag picture gives a natural explanation for this unusual behaviour.Comment: Revtex-file, 4 PRB pages + 5 figures attached as uu-encoded ps-files Hardcopies of figures (or the entire manuscript) can also be obtained by e-mailing to: [email protected]